i have it for ak47, it gives 45bullets instead of 30 i think. if you just use it normally (i.e. reload after every kill when it is safe) it is extremely good as the extra 15bullets can kill 1 or 2 enemies and then it gives you enough time to reload . it takes away the need for sleight of hand because you are less likely to need to reload in a dangerous situation.the 1 bad thing about it is that you can be easily drawn into the idea of spraying an area, causing alot of recoil making it harder to kill a target and it will use up lots of ammo.

it doesnt increase the amount of ammo you start with though, so scavenger is probably the best perk to use, because you get drawn into spraying.